In marine seismic exploration, a ship tows a powerful sound source and a streamer cable including arrays of hydrophones. At intervals along the survey line, the sound source generates acoustic waves which propagate through the water and into the underlying earth layers. The acoustic waves become reflected from the earth layers and return to the water surface where the acoustic waves are detected by the hydrophones. The hydrophones convert the acoustic waves to electrical signals which are transmitted through a multiconductor cable to a signal recording device on the ship. The streamer has a neutral or slightly positive buoyance. Typically, a streamer may be up to two miles long, contain a thousand or more hydrophones and 100 pairs of electrical conductors. A serious financial loss results when a portion of the streamer becomes severed from the tow.
It is customary to attach to the end of the streamer a tail buoy usually provided with a radio transmitter, a high-intensity flashing light beacon and even a radar reflector. By use of a direction finder and/or visual sighting of the beacon, a severed buoy can be located and then recovered. Also, since strong oceanic cross currents may sweep the far end of the very long streamer off the desired survey line, the direction finder can provide a continuous means to measure the off course deviation of the streamer's remote end.
Various types of tail buoys have been used in the past, including sleds, catamarans, can buoys, discus buoys, spar buoys, spherical rubber buoys, and wooden rafts. The desiderata for a suitable tail buoy are: it must be stable under tow at speeds of 6 to 10 knots: it must provide means to elevate and support a flasher, radio antenna, and radar reflector well above the water level; it should present minimal drag to the towing ship; it should generate minimal acoustic noise; it must be compact, light and yet sufficiently rugged to withstand rough seas, and it must be self righting.
Although sleds, catamarans, discus buoys and rafts are capable of supporting a mast for the radio and flasher, they are relatively heavy, bulky and sometimes capsize in heavy seas and fail to right themselves. Cylindrical can buoys and spherical rubber buoys are unsuitable for supporting a mast because the vertical axis is unstable unless a keel is added; but a keel adds weight, bulk, and generates undesirable noise and creates considerable towing drag.
Conventional spar buoys are satisfactory from the standpoint of weight and compactness, but they are unstable when under tow.
It is therefore an object of this invention to provide a light, compact, and rugged tail buoy which is stable, presents minimal drag, and generates minimal acoustic noise when under tow.